Bar code scanning systems are well known and widely used in supermarkets and other stores. These systems essentially operate by scanning a light beam (e.g., from a laser) over a bar code label (e.g., utilizing a universal product code ("UPC")) affixed to an item and identifying the item based upon the light scattered by the label. The code is associated with a product and price via a database. More specifically, UPC labels comprise a series of black and white stripes. When the laser shines light upon the UPC, the black stripes absorb most of the light and the white stripes scatter most of the light. The scattered light is sensed by a photodetector which generates a current that is typically on the order of 0.1 microamps.
In a fixed scanning system, the item bearing the UPC label is moved over an opening in a scanning surface. The light beam passes through the opening and, hopefully, intersects the UPC label in such a way that the UPC made be read. In order to increase the probability that the light beam intersects the UPC label, the direction of the light beam is altered by a set of mirrors that is constantly rotated.
The photodetector is physically positioned such that is gathers light scattered from the UPC label. The photodetector may also receive extraneous light, including light from intense sources such as sunlight.
One would expect the scanning system to be inoperative during any period that intense light (e.g., sunlight) is received by the photodetector. This is became when exposed to such intense light, the photodetector may produce a current of as high as .about.100 milliamps, which is on the order of one million times greater than the signal of interest generated by the photodetector in response to the light scattered by the UPC label. For this reason alone, existing bar code scanners are typically inoperative in the presence of bright sunlight. Another independent reason why bar code scanners are inoperative in the presence of bright sunlight is because noise contributions arising from the bright sunlight are about the same magnitude (0.1 microamp) as the signal of interest generated in response to the light scattered by the UPC label, making the light scattered by the UPC label difficult to interpret.
However, the problem with present scanning systems is that they are not only inoperative during the instant that intense light is encountered by the photodetector but in addition they take a long time to recover after an optical overload. An optical overload is considered to have occurred when the electrical system is driven beyond its design operating range. Consequently, present scanning systems remain disabled even during the time periods between intense periodic flashes of light. Thus, when the next intense flash of light occurs, the scanning system is disabled again. This occurs continually, due to the set of mirrors being rotated, and results in the scanning system being permanently held or "pinned" in an inoperative state as long as sunlight is present.
An attempt at solving this problem which has been made involves using a red piece of plastic to filter out extraneous light. However, even just the narrow bandwidth of red sunlight which passes through the filter is enough to pin the system in an inoperative state.
The problem with intense light is so acme that entire buildings (e.g., stores) are typically designed so that the bar code scanning devices will be exposed to minimal sunlight.